Synaptic plasticity, trophism, chemotaxis, interneuronal recognition, block of synaptic transmission, retinotectal projection. Retinal fibers relaying visual information to the tectum in goldfish are capable of dynamic modifications of their synaptic connections. Following a localized post-synaptic blockade of transmission, electrophysiological evidence suggests that the fibers move their terminals into nearby areas, where the cells are still responsive, in competition with fibers already there. This destabilization of blocked synapses suggests a possible trophic feedback signalling effective transmission. In addition, other experiments suggest a chemotactic factor which causes fibers to innervate any unoccupied territory. Following a localized removal of retinal fibers via application of a presynaptically acting toxin, surrounding healthy fibers invade the denervated area. The dual role of nerve growth factor (NGF) in the sympathetic nervous system suggests that these functions may be due to a single factor, released by the post-synaptic neuron in proportion to its activity and taken up by the presynaptic fiber in proportion to its activity. The proposed experiments will investigate the movements of terminals in the two cases both electrophysiologically and anatomically. In addition, they will examine: 1) the activity dependence of the movements of the terminals and 2) the ability of NGF and its antiserum to block the fiber movements out of the blocked area and into the denervated area respectively. Finally, the experiments will test for uptake and transport of labelled NGF by the retinal fibers and its dependence on activity.